Natural zeolites are hydrated aluminosilicates of alkali and alkaline earth metals. Zeolites have a crystalline structure commonly known as framework aluminosilicates with infinitely extending three dimensional networks of AlO4 and SiO4 tetrahedra linked to each other by the sharing of all oxygens. This three dimensional network structure provides extensive surface area within the zeolite, with up to 50% of zeolite volume attributable to the channels and cavities. This property contributes to a specific gravity that is intermediate to other mineral phases of the natural ore, which increases the difficulty of separations by prior art methods.
Natural zeolites are used in a variety of applications, including, for example, ion exchange, radioactive waste treatment, industrial waste treatment, uses as animal feed supplements, moisture absorbents, carriers for time-released substances such as pesticides or fertilizers, liquid and gas filters for contaminant and odor control, oil absorbents, and industrial coatings and fillers. Naturally occurring zeolites are also frequently used as starting material in synthetic zeolite production. Zeolites have also demonstrated usefulness as catalysts in hydrocarbon conversion reactions. The large surface area of zeolite makes it an excellent choice for such applications.
Another feature of zeolite structure is that the cavities within a particular zeolite are all of uniform shape and size. Consequently, natural zeolites may act as analogs to artificial molecular sieves.
One natural zeolite, clinoptilolite, possesses a particularly high absorbing capacity due to its large surface area. Furthermore, clinoptilolite offers a high cation exchange capacity, making it suitable for use in numerous industrial applications. The pore size of clinoptilolite makes this zeolite well suited to waste water filtration, particularly due to its demonstrated selectivity for strategic ions. Finally, the thermal and physical stability and compatibility with final waste forms, such as cement or glass, make it an attractive alternative to less stable and incompatible options such as polymer based ion exchange resins.
Naturally occurring zeolite ores are well known to contain a variety of contaminants, including, for example, clay, quartz, mica, feldspar, iron and titanium minerals and calcites. Naturally occurring zeolites have heretofore been effectively excluded from certain applications which require extreme brightness, such as in the fine paper industry where higher cost titanium dioxide, calcium carbonate or silica may be preferred additives. Furthermore, naturally-occurring zeolites are frequently passed over for use as molecular sieves or as catalysts because of ineffective purification methods.
Currently available and prior art process for extraction, purification and classification of natural zeolites are very limited and not commonly practiced. The art of clay minerals benificiation having been applied to zeolite ores includes pulverizing, dry classifying or wet gravitational separations, magnetic separation, bleaching and calcining to drive water out of the pores has proven relatively ineffective. A number of such processing techniques have been described in the prior art for zeolite applications, but have not been commercially successful. For example, in U.S. Pat. No. 4,510,254, a batch process is described in which a zeolitic ore is processed through the steps of pulverization, slurrying, removal of fines, fine milling, magnetic separation, bleaching and drying. The '254 process results in a dry finely ground zeolite having a particle size of below 2 microns and a TAPPI brightness of at least 90. Zeolite obtained from the '254 process also possesses a bulk density of about half or less of high quality kaolin clay pigments. Despite the claimed properties of zeolite obtained from the '254 process, natural zeolite deposits remain difficult to treat to sufficient purity, brightness, size discrimination and density. The overall yield of '254 process is less than 20% compared to greater than 40% for the present invention. Similarly, the zeolite content of the product obtained from the '254 process is slightly lower than the mineral source whereas the process of the present invention enhances the zeolite content by over 20%. Processes according to the prior art, including the '254 patent, typically include complex chemical methods that introduce undesirable chemical contamination as part of the purification process and generate large waste inventories having difficult and costly treatment requirements. Introduction of chemical contaminants further complicates the prior art processes by requiring intermediate steps to ameliorate the affect of such chemical contaminants. Some prior art classification techniques, such as that described in U.S. Pat. No. 5,603,411, also add undesirable chemical contaminants such as flocculating agents and dispersants. Consequently, artificially produced zeolites and polymeric resins remain the prime choice in high-end applications, such as ion exchange. Similarly, high cost minerals such as titanium dioxide and calcium carbonate are preferred over zeolite in fine paper manufacturing.
The absence of natural zeolite competition in various high end applications is largely an issue of effective and economic processes for extraction and purification from zeolite ores. There remains a need therefore, for a process which permits economical and technically sufficient exploitation of naturally occurring zeolite materials for a broad variety of applications. Furthermore, there is a need for a process which does not introduce chemical contaminants, such as flocculating or dispersing agents, and which does not produce hazardous process effluents.